The version of the Silver Kelolon that appears at the Numara Atoll are extremely, extremely evasive. They're also very quick and love to flee the battle; it's hard to defeat one using your traditional tactics even when your levels are in the 70s. You can defeat them earlier on though if you have someone cast Gamble and get lucky enough to kill them with it. Having another party user cast Casting Support on your Gambling character beforehand to speed them up. These guys are worth a good chunk of experience, and 8 SP each --which means they're one of the few enemies that you can really use to grind levels; not that it's necessary. It's actually a complete waste of time at this point.